A few things I've done that made me feel like it was a major achievement.

In battlefield 1942, I shot down an enemy fighter with my anti tank bazooka, while it was on a strafing run.

In Battlefield 2, I shot down an F-22 with my anti tank rocket launcher, while it was on a strafing run (similar achievement as the one stated before )

In many times when I played Rainbow Six (the 1st one), I've done 8 vs 8 team matches and I would score all 8 kills for that round.

This one I feel was totally awsome. I'm doing this tournament for Ace Combat 6 (didnt win the tournment sadly). It's a 2 on 2 dog fight. Me and my wingman focused on fighting 1 on 1, I would go after 1 guy and my wing man went after another. Well, our opponents started getting the advantage on one of us, I think my Wingman had to focus on being "defencesive". I'm telling my wingman to turn around and fly torwards me and fly straght for a few seconds. He's giving the okay, but we're still pretty far apart so he's doing some small maneuvers but more of less still flying in a straght line torwards me. I'm flying straght too, I'm baiting my opponent who's chasing me. As soon as my wingman passed me, I targetted the guy chasing him, fired my missiles and BLAMMO, got em! My wingman wasnt so swift, but he was able to scare off my persuer, but we later on ganged up on that guy and splashed him.

I was playing Blitzkrieg with a friend over LAN. He was an american and I was a german.

He destroyed my tanks with artillery strikes, generating confusion over my troops. After the bombardment was over, I was left with one Tiger II tank, while he had almost all of his initial tanks. He sent his troops forward and my single Tiger kept killing them all with one or two shots. I ended up destroying all of his tanks and apcs with one Tiger.

The only bit of pwning awesome I can really admit to is playing Dystopia (really great multiplayer mod using the source engine), accidentaly firing off a rocket at the rest of my team bunched together, somehow missing them and blowing up an unlucky cloaked scout.

Every ultra combo at the end of a Killer Instinct match in the original arcade ver.

Spending a month (summer at the grandmother's) with a friend who was summering at his grandparents' house next door, writing tons of notes and maps and codes until finally beating Metal Gear for the NES back in the day.

Oh, another one of mine was while playing Halo 3, team death-match on Valhalla, I pulled off a splatter on a guy who landed from the man-cannon by landing on top of him with a mongoose. I'm just sad it wasn't in ranked, but I did get it on film. XD